Condensation polymerization is well-known in the art, and is in widespread commercial use for the production of numerous polymers, most notably polyesters and polyamides, as described in Plastics Materials, 5th ed., by. J. A. Brydson, Butterworth-Heinemann, United Kingdom, 1989.
One example of a polycondensation reaction is the preparation of nylon 6,6. In well-known commercial practice, nylon 6,6 is prepared by polycondensation of hexamethylene diamine (HMD) with adipic acid. In the process, an aqueous salt solution of ca. 50/50 HMD and adipic acid is first prepared. A given salt solution can be characterized by a concentration of carboxyl ends and a concentration of amine ends, both in units of equivalents per 10.sup.6 grams of polymer. The ends balance is defined as the difference between the carboxyl ends concentration and the amine ends concentration. In typical practice, the solvent water is partially removed by evaporation, after which the solution is introduced to a reactor wherein additional solvent water is driven off along with water of reaction. Water of reaction must continue to be removed in order to drive the reaction to high molecular weight product.
In a typical commercial process, there is a narrow target range for the ends balance in the product so produced. Excessive variation in ends balance correlates with unacceptable variation in product properties. Maintaining the ends balance within the target range is thus an important aspect of nylon polymerization control. Ends balance is known to be altered by an alteration in the concentration of reactants or the occurrence of undesirable side reactions. In particular, it has long been known in the art that the water vapor vent gas streams contain some HMD, thus disturbing the balanced stoichiometry of the salt solution, resulting in undesired variations in molecular weight distribution, final polymer amine ends concentration, lower process yields, and more variable product than is theoretically possible. Trying to compensate for the lost HMD by addition of HMD during reaction without a knowledge of how much is being lost would merely substitute one problem for another. HMD during reaction without a knowledge of how much is being lost would merely substitute one problem for another.
It is further known in the art that certain undesirable side reactions occur during polymerization of nylon 6,6, which side reactions have a deleterious effect on productivity. The products of some such side reactions are volatile, and driven off in the water vapor vent gas stream. An example is cyclopentanone (CPK) which is formed from the thermal degradation of nylon 6,6.
Similar considerations to those hereinabove articulated in respect to nylon 6,6 apply to the formation of other polyamides produced by condensation of polymerizations involving HMD, such as nylon 6,10 or nylon 6,12, and to other condensation polymers including polyesters such as polyethylene terephthalate.
It is known in the art to use on-line real-time methods for analyzing hydrocarbon concentration in vent gas streams in certain small molecule chemical processes. It is further known in the art to use on-line real-time determinations of polymer product viscosity to provide input into closed loop process control of polymerization. However, the art has no teaching of any method for on-line real-time analysis of reactant concentration in a polymerization process nor of any means for achieving such analysis.
Rothstein (U.S. Pat. No. 3,754,125) broadly discloses means for closed loop process control of the reaction of a gas and a liquid wherein the gas feed is analyzed continuously for impurities and the gas feed rate adjusted to compensate therefor. Rothstein states explicitly that analysis of the vent gases is unsatisfactory because of the time lag between the occurrence of a process upset and the indication thereof in the composition of the vent gas stream, resulting in unacceptably high swings of impurity concentration within the reactor.
Ochiai (U.S. Pat. No. 3,972,946) discloses a process for conversion of alkenes to aldehydes and ketones by a catalyzed reaction with aqueous hydrochloric acid in the presence of cuprous chloride wherein the rate of addition of hydrochloric acid to the reaction is directly controlled by the continuous on-line determination of unreacted alkene in the vent gas flow from the reaction using infrared spectroscopy. Suitability of this configuration for closed loop process control is stated but no method therefor is described.
Amato et al. (U.S. Pat. No. 4,071,572) disclose a process for the oxyhydrochlorination of ethylene wherein the ethylene content of a vent gas stream is continuously monitored using infrared spectroscopy on the vent gas stream condensate, the data thereby provided used for adjusting the inflow of reactants to control the ethylene content in the vent gas stream within acceptable limits, thereby controlling the yield of the reaction.
Jordan (U.S. Pat. No. 4,630,038) describes an on-line automated self-calibrating analyzer employed to measure the hydrocarbon content of a vent gas stream from a solvent recovery unit. For Jordan's purposes, a continuous gas stream is provided to an on-line infrared analyzer capable of analyzing for numerous organic and inorganic species, including hydrocarbons. Data is stored and averaged over pre-set time intervals and displayed. The data storage device is also automatically interfaced with an alarm system providing immediate feedback should the hydrocarbon content exceed certain limits. A self calibration method involving the automated insertion of a blank and a known composition gas stream is provided.
Rao et al. (U.S. Pat. No. 4,952,345) disclose a process for spinning of polyamide. fibers wherein relative viscosity of the molten polymer is controlled by the humidity of the inert gas stream used to condition the polymer flake, the humidity being controlled by a feedback loop provided from continuous on-line determination of the melt viscosity of the polymer prior to spinning.
Buchelli (U.S. Pat. No. 5,065,336) describes a very generalized and theoretical means for computing the molecular weight distribution in real time of a polymer being formed in a continuous plug-flow polymerization reaction based upon online determination of polymer solution rheology. The results of the theoretical calculation can be employed in a closed-loop polymerization process control system to control molecular weight.
Laurent et al, (U.S. Pat. No. 5,155,184) describe a process for the manufacture of a polymer having at least one property P with a desired value D, the process being controlled by periodically sampling the polymer produced, determining the value of the property P by applying a correlation relationship with absorbance measurements in the near infrared, and then using the difference between the determined value of the property P and the desired value D to control the process parameters using a process computer.